1. Field of the Invention
The present invention relates to an organic light emitting display device, and in particular to an organic light emitting display device with compensation for deterioration of light emitting elements.
2. Discussion of Related Art
Recently, flat panel display devices of reduced weight and volume have been developed. Flat panel display device types include liquid crystal display devices, field emission display devices, plasma display panels, and organic light emitting display devices. Organic light emitting display devices display an image using organic light emitting diodes, which generate light by recombination of electrons and holes. Organic light emitting display devices have rapid response speeds and low power consumption.
FIG. 1 is a circuit diagram showing a pixel of a conventional organic light emitting display device. The pixel 4 of the conventional organic light emitting display device includes an organic light emitting diode OLED and a pixel circuit 2 coupled to a data line Dm and a scan line Sn to control the organic light emitting diode OLED.
An anode electrode of the organic light emitting diode OLED is coupled to a pixel circuit 2, and a cathode electrode of the organic light emitting diode OLED is coupled to a second power supply ELVSS. The organic light emitting diode OLED generates light with a brightness corresponding to current supplied from the pixel circuit 2.
When a scan signal is asserted on the scan line Sn, the pixel circuit 2 receives a data signal from the data line Dm to control an amount of current supplied to the organic light emitting diode OLED. To accomplish this, the pixel circuit 2 includes a first transistor M1″, a second transistor M2″, and a storage capacitor Cst. The second transistor M2″ is coupled between a power supply ELVDD and the organic light emitting diode OLED. The first transistor M1″ is coupled between the second transistor M2″ and the data line Dm and the scan line Sn. The storage capacitor Cst is coupled between a gate electrode and a source electrode of the second transistor M2″.
A gate electrode of the first transistor M1″ is coupled to the scan line Sn, and a first electrode of the first transistor M1″ is coupled to the data line Dm. A second electrode of the first transistor M1″ is coupled to a terminal of the storage capacitor Cst. The first electrode may be designated a source electrode or a drain electrode and the second electrode designated a drain electrode or a source electrode respectively. Formally, the designation of source electrode refers to the source of carriers in a transistor; however, transistor M1″ operates as a pass transistor so that there is no substantial distinction between source and drain. Hereinafter, “source electrode” or “drain electrode” will be used without elaboration. Those skilled in the art will appreciate the symmetry, general interchangeability, and accept the nomenclature for its conciseness. When the scan signal is asserted on the scan line Sn, first transistor M1″ is turned on to supply the data signal from the data line Dm to the storage capacitor Cst. The storage capacitor Cst is charged to a voltage corresponding to the data signal.
The gate electrode of second transistor M2″ is coupled to a terminal of the storage capacitor Cst, and the source electrode of second transistor M2″ is coupled to the other terminal of the storage capacitor Cst and the first power supply ELVDD. A drain electrode of second transistor M2″ is coupled to the anode electrode of the organic light emitting diode OLED. The second transistor M2″ controls an amount of current flowing from the first power supply ELVDD to the second power supply ELVSS through the organic light emitting diode OLED. The current corresponds to a voltage value stored in the storage capacitor Cst. The organic light emitting diode OLED generates light corresponding to the amount of current supplied from the second transistor M2″.
This conventional organic light emitting display suffers from reduced brightness over time. In other words, as the organic light emitting diode OLED deteriorates with time, the organic light emitting display device no longer displays an image with the desired brightness.